A problem encountered in many pneumatic tire systems involves maintaining the air pressure in the tires. Furthermore, when multiple sets of dual tires are used, low pressure in one of the tires may often go undetected for long periods of time because the other wheels and tires in the vehicle are sufficient to support the loads and frequently hinder access to inner wheels of the set.
While many drivers periodically check the tires of a vehicle during stop overs by tapping the tires with a tool, this precaution is sometimes overlooked. In any case, it is not effective for detecting low pressure in a tire during operation between checks. A driver often has no warning when the pressure in one of a number of tires drops below a safe operating level while he is driving the vehicle. Low pressures during operation may result from slow leaks in the tire, for example.
During operation, low pressure in one of the tires may cause any one of a number of serious problems. When pressure in one tire is too low, it may cause another tire to heat up and possibly cause a fire. Continued operation of the poorly inflated tire may also cause overheating, shredding, and destruction of the tire which, if the low pressure were detected and restored in time, could be recappable.
It is desirable that a tire pressure and maintenance system in a vehicle utilize a source of pressure already available. If for some reason the system is inoperable, it is also desirable to have independent means for inflating the tires and for detecting the pressure therein. Also, it is desirable to brake the vehicle to prevent movement thereof when a large leak in one of the tires develops.
In a patent entitled "Tire Pressure Control in a Vehicle", U.S. Pat. No. 4,387,931, issued June 14, 1983, and assigned to the same assignee as the present invention, there is described a tire pressure control system in which many of the problems discussed above have been solved to a great extent in a trailer system. In this application, tire pressure was applied to the tires through a hollow tubular structure which was located at the axis of the wheel and originated from a source of pressure located inboard of the wheels of the trailer. The arrangement illustrated, while applicable to trailers having hollow axle units, pose some problems when solid axles are used, as in a tractor, bus, or similar type vehicle.
A patent, U.S. Pat. No. 4,290,509 entitled "Anti-Lock System" describes a system for connecting air pressure through a wheel by means of a pressure path from outboard to inboard of the wheel and which is offset from the wheel axis as is necessary to solid axle vehicles, such as tractors. In this system, a pair of concentric sealing rings form a sealed chamber to connect a source of stationary air pressure located inboard of the wheel to an anti-lock switching device rotatable with the wheel outboard of the wheel. While this arrangement solves a problem of how to connect air pressure from a source through a wheel rotating on a solid axle, the sealing arrangement involving the rotating seal rings poses a problem.
In the system described in U.S. Pat. No. 4,387,931, the system incorporates "full time" pressurization of the tires. Rotating seals of the type described in the aforementioned anti-skid patent would present problems if they were used in tractors and full time pressurization of the tires was employed. The reason for this is that the temperature of the seal rings would rise greatly. Also the constant pressure of the seals would greatly reduce the life of the seals because of the constant high friction resistance involved.